Fuel composition may vary depending on the blending specifications for different regions based on climate and environmental regulations. Specifically, various additives may be added to fuel blends to alter fuel volatility based on the climate of the region where a fuel is sold. For example, fuels sold in southern areas with a warm climate may have a lower fuel volatility than fuels sold in northern areas with a cold climate so that the differences in climate corresponds to a difference in fuel volatility, thereby achieving a similar effect on emissions. Similarly, fuel volatility may vary throughout the year in a same region based on the climate of the region. For example, fuel dispensed at fuel pump may have a lower fuel volatility during warmer months than fuel dispensed during colder months. Furthermore, commercial fuel distributors may offer fuels comprising a blend of gasoline and ethanol (e.g., E10, E25, E85, etc.) to reduce carbon emissions. Further still, a fuel tank may be refueled with fuel of a particular composition while the fuel tank still contains some amount of fuel, possibly of a different composition. As a result, a typical fuel tank may contain a plurality of different fuel blends.
Meanwhile, environmental regulations mandate a decrease in vehicle emissions for vehicle manufacturers. As a result, vehicle control routines relating to engine operation, leak detection, and so on may depend on the combustion properties of a fuel to optimize engine efficiency and meet the environmental regulations. Furthermore, on-board diagnostic monitors of an engine control system also apply fuel volatility estimates, for example in the monitoring and detection of fuel system leaks. Reid vapor pressure (RVP), defined as the gauge pressure of a liquid fuel with a volume of air above it at a reference temperature (specifically, 100 degrees Fahrenheit), is typically used to estimate fuel volatility. RVP is a close estimate of the vapor pressure, which is an absolute pressure.
However, the relationship between vapor pressure and temperature is non-linear, and so two fuels with slight differences in RVP may have substantially different combustion properties at higher temperatures. As a result, even small errors in RVP estimation may lead to decreased engine efficiency and false results in fuel system leak detection tests, for example, thereby resulting in increased emissions.
One approach to resolving the issue of RVP estimation, at least in part, is to measure the absolute vapor pressure of a fuel at current operating temperatures. Aside from pressure variation due to elevation and flow, the pressure is uniform within a volume. The vapor pressure is set by the hottest surface in contact with the fluid. Placing a temperature sensor at the hottest point in the fuel system is difficult as temperature widely varies and the location of the hottest point is uncertain. Furthermore, a fuel system may intentionally operate at a vapor-liquid volume ratio of zero and so the fuel system is always above vapor pressure, thereby increasing the difficulty of accurately measuring the vapor pressure.
The inventors herein have recognized the above issues and have devised various approaches to address them. In particular, systems and methods for sensing fuel vapor pressure are provided. In one example, a method for a vehicle comprises: during an engine start after the engine has been off for at least a minimum duration, actively controlling fuel pressure in the fuel system to a vapor-liquid volume ratio greater than zero and then recording sensed fuel pressure and temperature in the fuel system. In this way, the vapor pressure of a fuel at a given temperature may be accurately measured during isothermal conditions, thereby improving an estimation of RVP. In turn, control methods regarding fuel injection, ignition timing, and emissions testing may be updated based on the improved RVP estimate, thereby increasing efficiency of engine operation and decreasing emissions.
In another example, a method comprises, pulsing a fuel pump responsive to an engine cold start, and determining a fuel vapor pressure versus temperature characteristic based on fuel pressure and temperature while the fuel pump is being pulsed in response to a reduction in DI pump volumetric efficiency. In this way, fuel volatility may be accurately determined and used for subsequent vehicle control routines, thereby improving engine efficiency and reducing emissions.
In another example, a fuel system for an engine comprises: a fuel tank containing fuel; a fuel pump positioned within the fuel tank and configured to pump the fuel to one or more fuel injectors coupled to the engine; a temperature sensor coupled to a fuel passage connecting the fuel pump to the one or more fuel injectors; a pressure sensor coupled to the fuel passage; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: actively control the fuel pump responsive to the engine turning on after the engine has been off for at least a minimum duration; and record a sensed temperature from the temperature sensor and a sensed pressure from the pressure sensor. In this way, the vapor pressure and temperature of a fuel may be measured at the hottest point in the fuel system, thereby providing an improved estimation of fuel volatility.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.